The present invention relates to a method for recuperating potential energy during a lowering operation of a load. The invention is especially applied in operation of a work machine.
The term “work machine” comprises different types of material handling vehicles like construction machines, such as a wheel loader, an excavator, a backhoe loader and a dump truck (such as an articulated hauler). A work machine is provided with a bucket, container or other type of work implement for carrying/transporting a load. Further terms frequently used for work machines are “earth-moving machinery”, “off-road work machines” and “construction equipment”.
In connection with transportation of heavy loads, e.g. in contracting work, work machines are frequently used. A work machine may be operated with large and heavy loads in areas where there are no roads, for example for transports in connection with road or tunnel building, sand pits, mines and similar environments.
The invention will be described below for a wheel loader. This should be regarded as a non-limiting example of a work machine. The wheel loader comprises a driveline for propelling the machine via the wheels. A power source, such as an internal combustion engine, and especially a diesel engine, is adapted to provide the power for propelling the wheel loader. The wheel loader further comprises a hydraulic system for performing certain work functions, such as lifting and tilting a work implement and steering the machine. The power source is also adapted to provide the power for controlling the hydraulic work functions. More specifically, one or more hydraulic pumps are driven by the power source in order to provide hydraulic actuators (such as hydraulic cylinders) with pressurized hydraulic fluid.
In order to recuperate potential energy, the hydraulic system may comprise a hydraulic machine which is adapted to function as both pump and motor. More precisely, the hydraulic machine functions as a pump in a lifting operation and supplies pressurized hydraulic fluid to the hydraulic cylinder. The hydraulic machine functions as a hydraulic motor in a lowering operation and is driven by a pressurized hydraulic fluid flow from the hydraulic cylinder. The lowering operation defines an energy recovery state.
It is desirable to achieve an energy recuperation method for a work machine, which creates conditions for an efficient recuperation of energy during a lowering operation of a load.
According to an aspect of the present invention, a method is provided for recuperating potential energy during a lowering operation of a load, wherein a hydraulic system is adapted to lift and lower the load, comprising the steps of                providing at least two energy recuperation modes,        selecting one of said modes in response to a current operating state, and        controlling the hydraulic system according to the selected mode.        
The term “load” here refers to the load exerted on the hydraulic system (especially on the hydraulic actuators) during the lowering operation, which load comprises a load resulting from the weight of a load arm assembly, which is adapted to lift and lower the load, and any external load (payload) carried by the load arm assembly.
Load actuation in different modes of operation creates conditions for hydraulically recuperating a greater portion of the mechanical load power.
Further, the method is designed for determining which of said at least two energy recuperation modes is most energy efficient and responsively selecting the most energy efficient recuperation mode. Further, the selection of energy recuperation mode is performed with respect to the constraints of the specific hydraulic system used with regard to a maximum system pressure etc. The selection of recuperation mode is for example made before initiating the lowering Operation.
According to a preferred embodiment, a first recuperation mode is associated to that a weight of the load is below a predetermined limit and a second recuperation mode is associated to that the load weight is above the predetermined limit.
For example, the predetermined limit represents a load state, in which a load arm assembly, which is adapted to lift and lower the load, is substantially free of any external load. In other words, the predetermined limit may correspond to a sum of the weight of the load arm assembly and a small additional weight corresponding to some stuck material in the load arm assembly etc.
According to a further preferred embodiment, a first recuperation mode is associated to that a load arm assembly, which is adapted to lift and lower the load, is lowered with substantially no external load.
Thus, in this case, only the load arm assembly is lowered after having dumped the external load in a raised position. Such operation is for example used in gravel handling. In gravel handling, the gravel is scooped up from ground level by means of a bucket, the bucket is thereafter raised and the collected gravel dumped in a raised position, for example on a container of a dump truck. The bucket is then returned (lowered) to the initial position for scooping up more gravel.
According to a further preferred embodiment, a second recuperation mode is associated to that a load arm assembly, which is adapted to lift and lower the load, is lowered with a substantial external load.
Such operation is applicable where an external load is collected from a raised position and lowered to a lowered position. This is for example the case in fork handling of pallets, wherein a pallet is collected from a shelf and lowered to ground level before transportation to a destination.
According to a further preferred embodiment, the load arm assembly comprises a work implement adapted to carry the external load. For example, the load arm assembly further comprises a boom, wherein the work implement (such as a bucket or forks) is connected to one end of the boom so that the work implement can be tilted relative to the boom.
According to a further preferred embodiment, the method comprises the step of detecting at least one operational parameter and determining the current operating state in response thereto.
According to one example of the last mentioned embodiment, a first operational parameter is indicative of a load state. Preferably, the current operating state is directly determined in response to the load state. Preferably, the first operational parameter is indicative of a pressure level in the hydraulic system.
According to a further example of the last mentioned embodiment, a second operational parameter is indicative of an operator commanded speed of the lowering motion.
Preferably, the current operating state is directly determined in response to the commanded speed. Especially, both the load state and the commanded speed are used as inputs for determining the operating state. For example, a position of an operator controlled element represents a commanded speed of the lowering motion. The aim is then to recuperate as much energy as possible given a desired actuator speed (commanded by the operator) at a given load acting on the actuator.
According to a further preferred embodiment, the method comprises the step of repeatedly detecting at least one operational parameter during operation in a repeated work cycle, and determining the current operating state based on detected values of the operational parameter during performance of at least one of said work cycles.
For example the current operating state is determined based on detected values of the operational parameter during performance of a plurality of said work cycles. The term “work cycle” comprises a movement of a work implement, such as a bucket, (lifting/lowering operation) and possibly any route of the work machine (ie the work cycle travel path) between a load collecting destination and a load release destination. The operational parameter is preferably only detected during the load lowering part of the work cycle. According to a first work cycle example, a wheel loader typically drives into a heap of material, lifts the bucket, reverses out of the heap, turns and is forwarded towards a dump truck where it unloads the material onto the container of the dump truck. After unloading, the bucket is lowered and the wheel loader returns to the starting position.
According to a further development of the last mentioned embodiment, the method comprises the step of repeatedly detecting said at least one operational parameter during operation in one of said at least two energy recuperation modes in the repeated work cycle, determining which of said at least two energy recuperation modes is most energy efficient for the specific work cycle, and responsively selecting the most energy efficient recuperation mode. The hydraulic system is controlled according to the selected energy recuperation mode in subsequent work cycles.
According to a preferred example, the hydraulic system comprises a hydraulic cylinder, which is configured to lift and lower the load, wherein the method comprises the step of controlling a flow from a piston side in the hydraulic cylinder during the lowering operation. Especially, a first energy recuperation mode involves controlling a flow communication between a piston-rod side and a piston side in the hydraulic cylinder during the lowering operation in a so-called differential mode. By using the differential mode, the flow to the pump may be reduced with about 30% with regard to a normal mode (in which there is no fluid flow connection between the piston-side and the piston-rod side). Thus, the pump size may be reduced. The differential mode causes a pressure increase in the system due to the area relationship in the cylinder. A relationship of 0.7 leads to a pressure increase with a factor in the magnitude of 3.3.
According to a first alternative, where higher lowering speed is required relative to what can be achieved in the normal mode due to the limited pump size, part of the flow may be throttled directly to tank if operation in the differential mode would result in a too high pressure level. This mode may be referred to as normal mode with meter-out flow control.
According to a second alternative, a valve is arranged in a line between the piston-side and the piston-rod side and the flow is throttled by means of the valve. In this way, the pressure level is limited. This mode is defined as a semi-differential mode. The power throttled away (pressure*flow) will be substantially smaller for high lowering speeds in the semi-differential mode compared to operation in the normal mode with meter-out flow control in case only a marginal pA-pB is required to stay below the maximum pressure level.
Similarly, in case of higher loads, the power throttled away (pressure*flow) will be substantially smaller in the normal mode with meter-out flow control where only a marginal meter out flow is required to achieve the desired lowering speed compared to operation in the semi-differential mode at the same operating point.
Further preferred embodiments of the invention are described in the following description.